Conventional electrical power distribution from the public utility to the end user typically incorporates wall sockets into which mating plugs can be inserted to complete the connection to a powered electrical apparatus. Under normal operations, each distribution socket would be "hot", meaning that electrical power in fact would be at the socket. Power outages at this distribution socket are possible due to many varied reasons; ultimately meaning however that with no electrical power at the socket, the powered apparatus would cease to function.
Because of possible consequences of extended power distribution outages, emergency power generators are quite common. A totally manual emergency power generator system would require an attendant, after the outage started, to start the generator and then switch the electrical connection of the electrical apparatus from the utility's distribution socket to the generator. A totally automatic system would sense when the outage started, and would then start the generator and switch the connection of the electrical apparatus responsive to this outage.
Inasmuch as the cause or timing of extended power distribution outages are not predictable, and having an attendant present to activate a manual system, totally automatic systems are preferred. However, such systems have generally required some form of hard-wiring into the utility's power distribution wiring and/or socket, and consequently, a skilled electrician. Special precautions are needed to preclude the possibility of connecting the generator's output and the utility's distribution lines together.